1. Field of the Invention
This invention relates, generally, to the field of generation of multivalued waveforms and, more particularly, to a tone generator to be utilized in a multitone multifrequency (MTMF) telephone station set.
2. Prior Art
The majority of telephone station sets in use today which incorporate multitone dialing generate the dialing signals with analog circuitry utilizing discrete components. Such circuitry typically utilizes tuned networks, comprising an inductance coil and a capacitor, each being tuned to one of the particular dialing tones required. Operation of any push button on a keyset causes, through electro-mechanical switching, two corresponding inductor-capacitor networks to be activated and coupled into resonant loops to generate the required component frequencies which are combined to form the multitone dialing signal.
Heretofore, use of such analog circuitry to generate multi-tone dialing signals has entailed certain limitations. For example, relatively large amounts of power are required to excite the two tuned circuits which are selected to generate the required frequencies. Thus, the number of push button multitone dialing sets that may be operated from one central office battery source is correspondingly limited.
Also, it is usually required that the telephone sets be usable despite relatively large temperature variations in the operating environment. Thus, the circuitry employed in the telephone sets to generate the multitone frequencies must maintain relatively constant electrical values throughout a broad range of temperature values. The high quality circuit elements required for generating the relatively error free frequencies required in this type of signaling results in higher manufacturing costs.
Moreover, since the tuned circuits are required to operate in the 700-1700 Hz range, components of rather large size are necessitated. Thus, a large amount of room must be provided in the telephone set for the analog circuitry required to generate the multitone dialing frequencies.
Recently, attempts have been made to utilize digital circuitry in the generation of multitone frequencies. One such circuit, disclosed in U.S. Pat. No. 3,787,836, issued to Hagelbarger uses controlled frequency division, shifting, delaying, and summation in suitable proportions to form approximations of the desired waveform. Such systems, however, require added filtering to remove harmonics generated. This added filtering substantially increases the expense of manufacture.
Digital circuitry, for the most part, can easily be implemented in integrated circuit form and thus manufactured in large volume with the advantages of low temperature sensitivity and small size. Further, such circuitry typically requires less power for operation.
Accordingly, this invention provides a solid state multitone signal generator, utilizing digital circuitry, which can replace existing analog multitone generation networks in use in present telephone sets. Moreover, such utilization of digital circuitry to provide a solid state signal generator will allow the invention to be fabricated in miniaturized form.
Additionally, this invention provides a multitone signal generator that may be operated on substantially less power than that presently required by the analog circuits in use in today's multitone telephone sets.